[DeTomaso] Brake Caliper Question

[JDeRyke at aol.com]

In a message dated 10/17/07 5:26:37 PM, loudonc at comcast.net writes:

> Thanks to all who responded on this. I apologize for not being clear. The 
> pistons can push back, but upon applying brake pedal pressure the pistons only 
> come back out a limited amount, not matter how much pressure is applied at 
> the pedal. I expected that the pistons would push the pads out.
> 
Varnish forms on brake pistons in all areas not directly touching the seals, 
and that varnish is   tenacious. In one rear caliper, I had to DRILL OUT the 
bleeder screw's passages to clear the varnish! What I do is to remove the pads 
and block all but one (1) piston from moving, with a wooden block between two 
opposing pistons and a single c-clamp holding #3. That leaves #4 piston in 
that caliper able to move. Then when you push hard, it WILL pop out of the 
caliper, making an almighty mess! Have a wide pan in position. I don't bother to pry 
the dust shield out since these are also usually crudded in place and tear 
easily. The piston will blow right through the rubber dust shield, not damaging 
it making the shield's removal easier afterwards- there's a spring-clip 
holding it to the caliper. 
If one piston still won't move, likely the shuttle valve has moved sideways 
and jammed, severely restricting fluid flow to one side. If so, the red 
indicator light should be lit up on the dash. Fix the shuttle valve first (or better 
yet, throw it away and replumb without it).
Scrub the caliper housing and piston with Scotch-brite to remove the varnish. 
Examine each piston very carefully as pits form that can cause leaks. The 
pistons are nickel plated mild steel, NOT stainless as in most U.S calipers, so 
water can penetrate the plating and rust can form. The seal is simply a 
square-section o-ring and is in the rebuild kits (with new dust shields) from all the 
vendors. Good luck- J Deryke



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